Abstract
The Pan-African Damara Belt is an example of a collisional orogen formed in a trench-trench-trench triple junction during the Gondwana Supercontinent assembly, as such, the Damara Belt records a complex tectonometamorphic history. As a result, there is ongoing debate around the number, timing, and duration of metamorphic events, and their relation to crustal anatexis, magmatism, and deformation. Of particular interest is the high-grade Central Zone of the Damara Belt, which exposes the lowest structural levels of the Damara Belt, characterised, among other things, by 10 km-scale migmatitic structural domes. This study aims to determine the tectonometamorphic and magmatic evolution of two of these domes, namely the Namibfontein-Vergenoeg (NV) domes, which are two adjacent, basement-cored domes in the southern parts of the Central Zone. To achieve this aim, an integrated, multi-disciplinary approach is taken whereby field-based structural geology is combined with petrography and thermodynamic modelling to determine the PT conditions of deformation. Additionally, U-Pb monazite geochronology coupled with monazite petrography and trace element chemistry (monazite petrochronology) of ten samples of structurally controlled granites and leucosomes is used to determine the timing of deformation and metamorphism at the NV domes.
An outcome of this study is a new geological map for the NV domes, based on remote sensing and field observations. The distribution of Abbabis Metamorphic Complex at the NV domes is confirmed by the recognition of diagnostic rock types and detrital zircon geochronology of basement meta-sedimentary rocks. This study recognises a magnetite-rich stromatic migmatite that only occurs in the pre-Damara Supergroup basement, which can be used as a marker of this unit in future fieldwork.
A detailed, field-based structural investigation of the NV domes shows that the NV domes formed through the superposition of four deformation phases in the presence of melt, giving rise to multi-fold interference patterns. Early E-W shortening produced steep N-S striking S1 planar fabrics linked to km-scale E-inclined F1 folds, likely reflecting the southern continuity of the fabrics of the Kaoko Belt. The superposition of NE-plunging, NNW-inclined F2 folds, formed during NNW-SSE, orogen-perpendicular shortening, on N-S trending F1 folds produced a type-1 dome-and-basin fold interference pattern and the general structural trend of the Damara Belt. Minor D3 deformation caused localised
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dome-scale folding of steep S1 and S2 into inclined to recumbent, NW-plunging F3 folds, and local top-to-the-S or -SE thrusting. D3 structures likely formed progressively during D2 NNW-SSE shortening to accommodate strain following F2 fold lock-up. NNW-SSE shortening is correlated with the collision between the Congo and Kalahari cratons that gave rise to the Damara Belt. Finally, orogen-parallel NE-SW shortening, at high angles to S2 fabrics, produced F4 folds with NE-dipping axial planes and NE-plunging fold axes. Orogen-parallel shortening in the Damara Belt is enigmatic and may reflect tectonic activities further afield in eastern and western Gondwana. The superposition of F4 folds on F2 folds defines a type-3 fold interference pattern that establishes the final 3D architecture of the NV domes. The presence of melt for the duration of polyphase deformation may have contributed to the complex structural inventory of the NV domes by weakening the rocks and making them more responsive to lower-stress events.
Petrography and pseudosection modelling of syn-kinematic mineral assemblages reveal that all deformation phases occurred at similar upper-amphibolite facies PT conditions in the presence of melt. Pseudosection modelling reveals a shallow (low-pressure-dominated) clockwise PT path for the rocks of the NV domes, with peak temperature conditions of 740-780°C and 3.5-6.0 kbar reached during late D1 and early D2. This study documents two periods of garnet growth, i) prograde metamorphic garnet that grew during heating of the system during D1 and D2, and a ii) peritectic garnet produced by incipient biotite dehydration melting during the thermal peak, commensurate with late D1 and early D2. The peak assemblage of Qz-Kfs-Pl-Bt-Sil-Grt-Crd-Ilm-Liq±Mag is noted for D1 and D2 samples. Cooling of the system down to the solidus along the retrograde path occurred during D4, where melt continued to form in the absence of garnet growth. Overall, rocks of the NV domes document a thermally dominated system with little change in pressure during their tectonometamorphic evolution.
U-Pb monazite geochronology combined with monazite petrography and trace element chemistry of ten structurally controlled granites and migmatite are used to constrain the timing of deformation and metamorphism at the NV domes. These data reveal that D1 E-W shortening was active at ~535 Ma, but not before ~555-561 Ma. D2 NNW-SSE, orogen-perpendicular shortening is dated at ~532 Ma, likely occurring during the waning stages of, or directly after D1 E-W shortening. D3, occurring progressively
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with D2 during NNW-SSE shortening must have been active before roughly 521 Ma. D4 NE-SW orogen-parallel shortening occurred at any time from ~523 to ~494 Ma. The geochronological record of syn-deformational granites likely indicates that the three tectonic shortening events that produced the four distinct deformation phases recorded at the NV domes all occurred directly after one another. Furthermore, monazite petrochronology documents that the continual resetting of monazite occurred alongside the crystallisation of cores, unzoned monazite, and monazite rims from ~550 Ma down to ~494 Ma. The total spread of monazite ages suggests that high-temperature anatectic and/or near-anatectic conditions persisted for ~80 m.y. (~575-490 Ma) at the NV domes, indicating that the rocks of the NV domes likely experienced one prolonged phase of HT metamorphism. Lastly, this study documents the occurrence of some of the oldest S-type granites in the Central Zone of the Damara Belt, at 561 ± 6 Ma, 558 ± 7 Ma, and 555 ± 6 Ma.
Overall, these data suggest that the rocks of the NV domes were deformed at least four times during three consecutive and closely timed regional shortening events related to large-scale tectonic movements. This polydeformation occurred at suprasolidus conditions during prolonged, HT metamorphism, which has implications for local and regional tectonics in the Damara Belt. The presence of melt during deformation may have affected the local structural inventory of the NV domes and could explain the contrasting structural records and therefore proposed dome-forming mechanisms for structural domes in the Central Zone. Despite the rocks of the NV domes, and by extension, parts of the Central Zone, being hot and weak for almost the entire lifespan of the orogen, they were coherent enough to preserve polyphase deformation caused by multiple tectonic switches that reflect the movements and organisation of cratonic blocks during Gondwana Supercontinent assembly.